Surgical access to the brain for neurosurgical procedures is created by removing a portion of the patient's skull to provide an exposed area of dura and permit access to the soft tissue of the brain for surgery, a procedure termed a craniotomy. The craniotomy is determined by the location of the pathology within the brain, the safest/easiest access route and the degree of exposure required for the procedure. Once the location is determined, the first step is to create an initial perforation of the full thickness of the skull. Special skull perforators are available to create perfectly round holes but most surgeons simply use a rounded, end-cutting burr to create the perforation. Typically the perforation is in the range of about 11-15 millimeters (mm) in diameter. A surgeon may choose to create more than one perforation around the perimeter of the planned craniotomy. Some surgeons prefer a single perforation and others use more than one, but there is no standard number. Once this hole is created, it allows the insertion of a rotary powered surgical instrument (e.g., a craniotome) which is used to create a continuous cut (also called an osteotomy cut or a kerf) around the perimeter of the craniotomy. This kerf begins and ends at the perforation when there is one perforation or it runs from one perforation to another when more than one perforation is made in the skull. The kerf is made with a side cutting burr which is shielded from the dura (outer covering of the brain) by a foot plate on the craniotome. The foot plate extends below and forward of the cutting burr and the surgeon keeps the tip of the foot plate in contact with the inner surface of the skull as he performs the craniotomy. The typical kerf is made freehand with an approximately 2 mm diameter burr. The shape of the craniotomy is therefore highly variable and the kerf is not always oriented perpendicular to the skull. The kerf may be larger than 2 mm in some areas as well. Over the course of the kerf, the skull thickness will vary, typically over the range of 3-8 mm in adults.
Once the cut is complete, the skull flap is removed from the skull and placed on the sterile back table for reinsertion at the end of the procedure. The dura is then cut to provide access to the soft tissue of the brain. After completion of the soft tissue surgery (typically 1-6 hours), the dura is sutured back together, and the skull flap then is inserted back into the craniotomy and fixated to prevent movement and restore the original contour of the skull. The surgeon may bias the skull flap toward one side or another to create bone-to-bone contact in a particular area or he may leave a gap around the entire flap. The scalp is then closed and the patient is sent to the neurosurgical intensive care unit for recovery.
If complications develop while the patient is in the hospital, there may be the need for emergency access to the brain through the craniotomy site. In addition, some patients may return for subsequent craniotomies in the same region, particularly in cases of recurrent tumors. Postoperative imaging studies (MRI or CT) are generally conducted on all patients. There is no clear evidence that the skull flap ever completely heals (solid bony union) in adults. It is more likely that a combination of new bone formation and fibrous connective tissue fills the gap between the skull and the skull flap.
From a surgeon's perspective, the method of reattaching the bone flap must be safe, simple to use, be rapidly applied, permit emergent re-entry, not interfere with postoperative imaging studies, provide stable fixation and have an acceptably low profile. The ideal method would result in complete fusion of the bone flap to the native skull with no long term evidence of prior surgery.
Current methods of reattaching the skull flap include drilling a series of small holes in the edge of the skull and the edge of the flap. Sutures are then passed through the corresponding holes and the flap is secured back into the skull opening from which it was taken. Because the fit is not exact due to the material removed by the craniotome, the flap can sag and sit slightly below the surface of the skull resulting in a depressed area that is obvious through the skin.
Another common reattachment method substitutes stainless steel wire for the suture material and fewer holes are used. There is still the risk of a cosmetically objectionable depressed area resulting. Metallic cranial fixation is (generally) only ever removed if it becomes symptomatic or if it interferes with subsequent surgeries.
More recently, surgeons have begun to use the titanium micro plates and screws that were developed for internal fixation of facial and finger bones. While this method results in a more stable and cosmetic result, it is relatively expensive, does not insure fusion and leaves foreign bodies at the surgical site.
All of these methods take ten minutes to one hour of additional surgery after the soft tissue (brain) surgery.
There is another method in which a titanium rivet (or clamp) is placed inside the skull with the stem of the rivet (clamp) passing between the skull and the flap. A large “pop rivet” type tool is used to force an upper titanium button down over the stem of the rivet, locking the flap and the skull in place between the upper and lower buttons. Three or four of these rivets and buttons are used to secure the flap in place. This method can be faster than other methods and less expensive than the titanium plates, but more expensive than sutures or wires. Just as with titanium plates and screws, fusion is not assured and foreign bodies remain in the patient.
According to the present invention we have developed new systems for, and methods of, reattaching a skull flap in a skull opening. The fixation provided utilizing a fastener and dura cover of the invention and practicing the methods of the invention is secure and cosmetically acceptable. The fastener and dura cover can be used in combination with materials which can enhance bone growth in a manner which causes healing by means of bone-to-bone reattachment of the skull flap to the skull.
The term “perimeter contour” as used herein with reference to the skull flap means “the shape of the outer boundary around the skull flap.”
The term “shaped to follow the perimeter contour” means “formed around the perimeter contour.”